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Flickering, flashing and strobing effects on webpages can cause some people to have photosensitive seizures.

How Common is Photosensitive Epilepsy?

According to the National Society for Epilepsy, â€œone in 131 people have epilepsy and of these people, up to 5% have photosensitive epilepsy.â€ This means that for every 10,000 readers or visitors to your site, four or five people could have a tonic clonic (convulsive) seizure caused by flashing ads, animated GIFs or flashing red text.

What are the Recommendations for Bloggers, Internet Marketers and Web Designers?

The two guidelines related to making web content safe for people with photosensitive epilepsy are:

The US legislation Section 508 of the Rehabilitation Act, which applies to all Federal agencies, states in subsection 1194.22 Pages shall be designed to avoid causing the screen to flicker with a frequency greater than 2 Hz and lower than 55 Hz.

How Can I Test Flickering Speeds?

Once installed, click on Images (keyboard shortcut: alt + 4) on the toolbar, then select GIF Flicker Test. Youâ€™ll be presented with a basic report indicating which GIFs have flicker rates within the range that may affect people with photosensitive epilepsy.

For a more in-depth analysis, check out the Trace Center’s Photosensitive Epilepsy Analysis Tool (PEAT) – a free, downloadable resource for developers to identify seizure risks in their web content and software.

What is the Bottom Line?

Even if the object does not cause a seizure, it may cause nausea or dizziness in some people. Neither of these is as serious of a health risk as a full-blown seizure, but having users mentally associate your web site with feelings of nausea is probably not the best design decision, at least in terms of user satisfaction and repeat visits.

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Comment by Alexander Schmidt

May 25, 2010 @ 12:56 pm

Thanks a lot for all the valuable information!

As I live and do a lot of projects in Germany, I have to design and implement sites (or full web applications) according to the German law. But the law is neither very strict, nor very “complete”. So, as far I am informed, epilepsy is not part of the law at all, for example.

But as I try to implement every project (not just those which fall under this law) as accessible as possible, I should really hire an expert like you to explain and teach me about how people with various handicaps approach my sites, to deliver a great experience for everybody.

You know, I really care about the people visiting my sites, but for a person considered “normal” it’s not that easy to see things from the perspective of a person labeled “handicapped”. I also don’t work for a big company like Yahoo and so don’t have a special department where I can test the things I’m building with special goggles to simulate seeing-restraints for example.

I also don’t like the words handicapped and disabled. Are there better words to describe somebody with disabilities? (I’m not a native english speaker and struggle to find better words.)

May 25, 2010 @ 1:21 pm

In reply to Alexander:

I believe that the German Federal Government has obligations to ensure that public websites are accessible, and the Web Content Accessibility Guidelines are considered to be probably the best way of measuring accessibility, and as Glenda says photo-sensitive epilepsy is covered in some detail.
That doesn’t necessarily mean that the same applies to non-goverment owned website though, I don’t know if there is any German equivalent to the ADA (Americans with Disabilities Act) or the DDA (Disability Discrimination Act – UK).
When you talk about not having special goggles, there are some tools available that can simulate the different forms of colour blindness which may help you (just search on Google).
I know that in the UK at least “handicapped” is no longer considered appropriate but “disabled” is in general usage (just look at the names of the laws for example).

Comment by Glenda

May 25, 2010 @ 1:47 pm

Alexander,

I’m glad you found the post useful. And, hats off to you for making your projects as accessible as possible!

You raise a good point about not having an in-house department for accessibility testing. Sounds like a toolbox of online testers and simulators might come i handy. And, yes, I’m also available to assist you on a contract basis, if you ever need me.

Regarding your question about language, by putting people first, like you said with “somebody with disabilities”, you said it perfectly! Other examples are “a woman with cerebral palsy”, “a teen who uses a wheelchair”, “perhaps with sight impairments”.

Your thoughts and questions are always welcome here.

May 25, 2010 @ 2:27 pm

There are thresholds below which flashing content will not likely cause a seizure. It must be sufficiently big, bright, and strobing more than 3 times per second. The color red is more likely to cause issues.

Most advertisements are not of sufficient size or contrast to cause seizures. However, they will be distracting and annoying to everyone. You must also consider someone with low vision AND photo-sensitive epilepsy. If they enlarge the flashing advertisement it could easily meet the size thresholds.

Comment by Alexander Schmidt

May 25, 2010 @ 2:59 pm

Thanks for your replies.

The point was – which Glenda picked up perfectly -, that for me and other people in my profession (developer, interface designer) it is often hard to understand our users in general. Admitting that leads us to (expensive) user-testing. All theory doesn’t help if your users interact with the software in a way you didn’t imagine, get stuck, do funny things when it’s unresponsive and so on.

A lot of effort goes into changing a complete product to be accessible. But like performance, it should be considered from the very beginning in order to deliver a great experience. So if there would be a user group with different disabilities, willing to be filmed (or observed otherwise) while using their tools necessary for interacting with a website, that would be incredibly helpful.

I am aware of the tools for developers to simulate things like color blindness. But I don’t want to build a “compatible” product, I want to build a product that is fun to use for everyone – no exception.

On a side note about Germany and accessibility:
Indeed there are efforts leading to more accessibility, but these aren’t enough in my opinion. We have one law regarding accessibility in information technologie (called BITV) issued in 2002 which is based on the WCAG 1.0 from 1999. Since 2002 (web)technology has changed a lot, things like AJAX call for WAI-ARIA. BITV says something about the need to be updated as technology evolves and BITV 2 exists as a draft for one and a half year but is still not happening.

But, you know, it’s not about a law forcing me to do something. It’s about me wanting do deliver a great experience for everyone. So watch out Glenda, I’ll contact you for sure!

May 28, 2010 @ 7:21 am

Flashing things are not just a problem for those with photosensitive epilepsy. In fact, although this is the most dramatic problems I think it’s probably the smallest group affected negatively by flashing content!

Just to clarify, all of this information is anecdotal – I’ve spent 15 years heavily involved in online chronic illness/disability communities as well as dealing with these things myself, but I don’t have statistics or hard data unfortunately. I don’t think any exists, to be honest! So YMMV but this is what I’ve been told and experienced:

Some people with migraines have migraines triggered by flashing advertisements.

Some people with certain neurological impairments (especially those that make it hard to mentally focus on things) find that their eyes are strongly drawn to things on the screen that are moving – sometimes so strongly that they’re completely unable to read and comprehend the text on the website. At some points I had this effect with my own neurological problems, although thanks to medications this effect is much less now.

It seems to me that a fairly large percentage of those who have diffuse neurological problems broadly named “brainfog” (many people with autoimmune diseases have this problem, as well as other chronically ill populations) or “chemobrain” (when it’s caused by chemotherapy) or “ms brain” (when caused by multiple sclerosis) or traumatic brain injury have these focussing and distraction issues, as well as people with ADD, AHDH, and autism-related issues.

Personally, I still find it very difficult to concentrate when there’s anything flashy on the screen so I use tools in Firefox which block things that commonly animate in distracting/flashing ways (animated GIFs and Flash based content). If this isn’t enough I also have bookmarklets that can remove all formatting/images/etc. similarly to the “Readability” bookmarklets.

Cheers,
r

Comment by deborah

May 31, 2013 @ 8:17 am

Recently a web ad for PC Performer is everywhere with fast, brightly, flashing ads and causing me lot’s of trouble. (epileptic) I may have to get off the internet because of it. Is anyone else having issues with this companies ads?